The photoelectric conversion element capable of converting light into electric energy can be employed in solar cells, image sensors, and the like. In particular, there has widely been used an image sensor in which a current generated by incident light in a photoelectric conversion element is read out by CCD and CMOS circuits.
In an image sensor using a photoelectric conversion element, an inorganic substance has hitherto been employed as a material composing the photoelectric conversion film. However, the inorganic substance cannot absorb only light having a specific wavelength because of its low color selectivity. Therefore, there was a need that a color filter is provided before a photoelectric conversion film to selectively transmit only specific colors (for example, red, green, and blue colors) from incident light. However, during imaging a fine object, use of the color filter may lead to an interference between the pitch of the object and that of an image element, thus generating an image which is different from an original image (Moire defects). When an optical lens is used so as to suppress the generation of the defects, there arises a problem such as deterioration of efficiency for light utilization and an aperture ratio.
Meanwhile, growing demands for higher resolution of the image sensor create an opportunity of the progress of microfabrication of pixels. Therefore, the size of pixels tends to decrease. Reduction in size of pixels leads to a decrease in quantity of light which reaches the photoelectric conversion element of each pixel, thus causing deterioration of sensitivity.
To solve these problems, a study has been made of a photoelectric conversion element using an organic compound.
Since the organic compound can selectively absorb light in a specific wavelength region of light being incident according to a molecular structure, it becomes unnecessary for the photoelectric conversion element using an organic compound to use a color filter. It is also possible to enhance efficiency for light utilization because of high absorption coefficient. There have been known, as the photoelectric conversion element using an organic compound, specifically, element structures in which a p-n junction structure and a bulk heterojunction structure are introduced into a photoelectric conversion film sandwiched between an anode and a cathode (see, for example, Patent Literatures 1 to 3). There has also been known an element structure in which a charge blocking layer is inserted so as to reduce a dark current (see, for example, Patent Literature 4).